Transmission of digital information presents many challenges. One issue concerns recovering timing information about the transmitted signal. This is necessary in order to properly sample the signal at the receiver. Some other issues are related to the effects produced on the signal as a result of the channel characteristics. The channel in a digital communication system may be characterized as a filter with a limited bandwidth. As a result, a square wave pulse inserted at a transmit end of the channel suffers from roll-off at a receiving end of the channel. In addition, the channel may also be plagued with multipath issues that manifest themselves as reflections of the original signal being added to the original signal. As a result of these characteristics, there may be a significant amount of inter-symbol-interference (ISI) seen at the receiving end of the channel. That is the individual symbols or bits begin to overlap one another as a result of reflections and roll-off produced by the band limiting and other effects of the channel.
To compensate for the effects of the channel an equalizer may be used. The equalizer may be based on a finite-impulse-response (FIR) filter adapted by a least mean squares technique (LMS). This is sometimes referred to as the time-domain LMS technique. This technique may have the advantage of operating without imposing any significant delay between the input and output signal. However, this quick response time is obtained at the expense of a computationally demanding process that becomes more demanding as the data rates increase.
When the data rates are higher, for example in the 10 Gbit/sec range, the time-domain LMS technique may become too computationally demanding and perhaps too costly. One method for dealing with the issue may be to perform the equalizer function in the frequency-domain rather than in the time-domain. A method that employs this technique is the constrained frequency domain block LMS algorithm. The problem with this technique, however, is that it does not have any constraint for the pre-cursor. It will therefore, adapt the pre-cursor tap-weight as other tap-weights. This will result in a non-zero contribution during the frequency domain FIR computation and will thus result in the loss of accurate timing error information that could be derived based on the zero-crossing at the pre-cursor that is introduced by the pre-filter.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.